It is broadly known that there are video door systems in the market that comprise at least one street panel with a video camera that communicates with a plurality of home terminals provided with an intercom and/or image display monitor by means of a non-polarized two-wire communications line to transmit audio, video, and data through the same, such that they are capable of supporting a conversation between two elements of the installation, transmitting two audio channels (upstream audio and downstream audio), a video channel (normally upstream), and a data channel to manage the system through said two wires. Low-cost elements with very few features (audio communication only) must coexist with other elements having many features (audio and video communication), for which cost is not a limitation, in this type of systems.
For example, we have invention patent ES2315138B1, which describes a system of the aforementioned type using distribution elements that allow creating a point-by-point corridor between the street panel and the home monitor. An FM modulation of the different analog signals is used in this system by means of different signal carriers or frequencies for each one of the signals (upstream audio, downstream audio, video, and data). In addition, a FSK modulation is used for data.
Invention patent U.S. Pat. No. 5,923,363 describes a similar FM modulation system; however, the installation is not really a bus installation, given that it carries out the switching by means of exchangers, resulting in a star installation. In addition, it requires batteries at the terminals to provide the energy supply that the cable is not able to provide to the data at the same time.
European patent EP1569454A2 describes a bus system that allows a single conversation channel, given that the video channel is not modulated. The upstream and downstream audio channels are modulated in FM. It also uses two infrastructure elements: a negative load simulator next to the supply source, which must be adjusted depending on the number of terminals present, and another simulator at the last terminal of each branch, which makes the installation more difficult.
European patent EP1843590A1 describes a two-wire bus video door system that requires the two monitors to be locally supplied at the home. Therefore, this system is not a comprehensive two-wire system, given that the supply for the monitor is not provided by the bus.
All the previous systems allow establishing a single simultaneous communication by means of the use of the analog modulation of information. Establishing more than one simultaneous communication between sets of elements of the system is not possible.
On the other hand, there are many multichannel video door systems in the market using a greater number of conductors. In this sense, we have IP video door systems, such as the one provided in patent ES2273578, which uses structured cabling and computer switch-type infrastructure devices. There are other systems that use coaxial cabling (more expensive) to transmit the different channels through different carriers, therefore requiring transmission means with large bandwidths.
In the state of the art of modulation systems, there are systems and patents that use analog modulation (AM, FM, PM) to transmit analog signals, and others that use digital modulation (FSK, PSK, PIM, PPM, TSK). With respect to the latter, the use of Pulse Position Modulations (PPM) allows a broad bandwidth and are usually used in wireless, optical fiber, or laser communications systems. In this sense, we can cite patent documents U.S. Pat. No. 6,711,122 and US2009010321, wherein the difficulty in transmitting the information through a uncharacterized two-wire line does not take place. All these cases use clearly characterized transmission means that do not hinder the transmission of information. Patent document WO2004090833A1 can also be cited, which describes a data transmission system through a two-wire line for a field bus supplying the devices. However, it has the following limitation: transmission rates are very low (unsuitable for the transmission of audio, never mind video), it uses a master-slave system that slows communication down because slaves need to be questioned one by one to see if they require transmitting information, it requires calibrating the slave devices continuously for their synchronization, and lastly, the changes in consumption of the devices are only allowed in certain moments when data transmission is not allowed, given that the information is affected.
Therefore, the current state of the art currently fails to offer a solution that allows transmitting several simultaneous conversations of audio and video through two conducting wires, through which management data and the power supply are also transmitted. In addition, the systems, still limited to a single conversation channel, require additional infrastructure to adapt the environment and facilitate the transmission of information, which depends on the number of installed devices and the topology of the installation. These system also require a specific type of cable and cannot be adapted to different types of cable.
The solution described below solves the limitations exposed above by means of a video door system that allows avoiding bus infrastructure elements, may be adapted to any type of cable and topology, and allows having simultaneous conversations in this environment and at a reasonable cost for the applicable video door system sector, wherein low-cost elements with very few features (audio communication only) must coexist with other elements having many features (audio and video communication), for which cost is not a limitation.